Process for the manufacture of metal nanoparticle

ABSTRACT

A process and apparatus prepares and collects metal nanoparticles by forming a vapor of a metal that is solid at room temperature, the vapor of the metal being provided in an inert gaseous carrying medium. At least some of the metal is solidified within the gaseous stream. The gaseous stream and metal material is moved in a gaseous carrying environment into or through a dry mechanical pumping system. While the particles are within the dry mechanical pumping system or after the nanoparticles have moved through the dry pumping system, the vaporized metal material and nanoparticles are contacted with an inert liquid collecting medium.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] Metal particles find a wide range of use as fillers, activemedia, explosives, magnetically sensitive materials, decorativematerials, taggants, and reflective material. The present inventionrelates to the field of metal nanoparticle manufacture and apparatus forthe manufacture of nanoparticles.

[0003] 2. Background of the Art

[0004] Many processes are available for the manufacture of small metalparticles. These processes cover a wide range of technologies andexhibit a wide range of efficiencies. Some processes produce dryparticles, while other processes produce particles in liquiddispersions.

[0005] Numerous references have appeared describing use of the gasevaporation technique to produce ultrafine metal powders, especiallymagnetic metal/metal oxide powders (often referred to as magneticpigments). These appear to exclusively refer to a dry process and do notinvolve contact with liquids. Yatsuya et al., Jpn. J. Appl. Phys., 13,749 (1974), involves evaporation of metals onto a thin film of ahydrocarbon oil (VEROS technique) and is similar to Kimura (supra).Nakatani et al., J. Magn. Magn. Mater., 65, 261 (1987), describe aprocess in which surface active agents stabilize a dispersion of aferromagnetic metal (Fe, Co, or Ni) vaporized directly into ahydrocarbon oil to give a ferrofluid using a metal atom technique. Themetal atom technique requires high vacuum (pressures less than 10−3torr) such that discrete metal atoms impinge onto the surface of adispersing medium before the metal atoms have a chance to contact asecond species in the gas phase. In this metal atom process, nucleationand particle growth occur in the dispersing medium, not in the gasphase. Thus, particle size is dependent on the dispersing medium and isnot easily controlled. Additionally, U.S. Pat. No. 4,576,725 describes aprocess for making magnetic fluids which involves vaporization of aferromagnetic metal, adiabatic expansion of the metal vapor and an inertgas through a cooling nozzle to condense the metal and form small metalparticles, and impingement of the particles at high velocity onto thesurface of a base liquid.

[0006] Kimura and Bandow, Bull. Chem. Soc. Japan, 56, 3578 (1983)disclose the non-mechanical dispersing of fine metal particles. Thismethod for prepares colloidal metal dispersions in nonaqueous media alsouses a gas evaporation technique. General references by C. Hayashi onultrafine metal particles and the gas evaporation technique can be foundin Physics Today, December 1987, p. 44 and J. Vac. Sci. and Tech., A5,p. 1375 (1987).

[0007] EPA 209403 (Toyatoma) describes a process for preparing dryultrafine particles of organic compounds using a gas evaporation method.The ultrafine particles, having increased hydrophilicity, are taught tobe dispersible in aqueous media. Particle sizes obtained are from 500Angstroms to 4 micrometers. These particles are dispersed by ultrasoundto provide mechanical energy that breaks up aggregates, a practice thatin itself is known in the art. The resulting dispersions have improvedstability towards flocculation.

[0008] Other references for dispersing materials that are delivered to adispersing medium by means of a gas stream include U.S. Pat. No.1,509,824, which describes introduction of a molecularly dispersedmaterial, generated either by vaporization or atomization, from apressurized gas stream into a liquid medium such that condensation ofthe dispersed material occurs in the liquid. Therefore, particle growthoccurs in the dispersing medium, not in the gas phase, as describedabove. Furthermore, the examples given are all materials in theirelemental form and all of which have appreciable vapor pressures at roomtemperature.

[0009] U.S. Pat. No. 5,030,669 describes a method consisting essentiallyof the steps: (a) vaporizing a nonelemental pigment or precursor to anonelemental pigment in the presence of a nonreactive gas stream toprovide ultrafine nonelemental pigment particles or precursor tononelemental pigment particles; (b) when precursor particles to anonelemental pigment are present, providing a second gas capable ofreacting with the ultrafine precursor particles to a nonelementalpigment and reacting the second gas with the ultrafine precursorparticles to a nonelemental pigment to provide ultrafine nonelementalpigment particles; (c) transporting the ultrafine nonelemental pigmentparticles in said gas stream to a dispersing medium, to provide adispersion of nonelemental pigment particles in the medium, theparticles having an average diameter size of less than 0.1 micrometer;wherein the method takes place in a reactor under subatmosphericpressure in the range of 0.001 to 300 torr.

[0010] U.S. Pat. No. 5,106,533 provides a nonaqueous dispersioncomprising pigment particles having an average size (diameter) of lessthan 0.1 micrometer dispersed in an organic medium. That inventionprovides an aqueous dispersion comprising certain classes of inorganicpigment particles having an average size (diameter) of less than 0.1micrometer dispersed in a water or water-containing medium. Thedispersions require less time for preparation, are more stable, have amore uniform size distribution, a smaller number average particlediameter, fewer surface asperities, and avoid contamination of dispersedmaterial due to the presence of milling media and the wear of mechanicalparts, these problems having been noted above for dispersions preparedby conventional methods employing mechanical grinding of particulates.Additionally, no chemical pretreatment of the pigment is required inorder to achieve the fine particle sizes obtained in the finaldispersion. The pigments of the dispersions are found to have narrowersize distributions (standard deviations generally being in the range of±0.5 x, where x is the mean number average particle diameter), are moreresistant to flocculation (i.e., the dispersions are stable, that isthey are substantially free of settled particles, that is, no more than10% of the particles settle out for at least 12 hours at 25° C.), anddemonstrate superior overall stability and color as demonstrated by lackof turbidity, by increased transparency, and by greater tinctorialstrength, compared to mechanically dispersed pigment dispersions.Furthermore, the method requires no mechanical energy, such asultrasound, to break up aggregates. Aggregates do not form since thereis no isolation of dry ultrafine pigment particles prior to contactingthe dispersing medium. The dispersions of any organic or inorganicpigment or dispersion that can be generated from a pigment precursor,are prepared by a gas evaporation technique which generates ultrafinepigment particles. Bulk pigment is heated under reduced pressure untilvaporization occurs. The pigment vaporizes in the presence of a gasstream wherein the gas preferably is inert (nonreactive), although anygas that does not react with the pigment may be used. The ultrafinepigment particles are transported to a liquid dispersing medium by thegas stream and deposited therein by bubbling the gas stream into orimpinging the gas stream onto the dispersing medium.

[0011] U.S. Pat. No. 6,267,942 describes a process for manufacture ofspherical silica particles. Silica gel particles to be dispersed in amixed solution of an alkali silicate and an acid are required to have anaverage particle size of from 0.05 to 3.0 micrometers. In a case wherethe average particle size of the silica gel particles is smaller than0.05 micrometers, mechanical strength of the spherical silica particlesto be obtained will be low, and irregular particles are likely to form,such being unsuitable. Similarly, in a case where the average particlesize of the silica gel particles is larger than 3.0 micrometers,mechanical strength of the spherical silica particles to be obtainedwill be low, and irregular particles are likely to form, such beingunsuitable. The more preferred range of the average particle size of thesilica gel particles is from 0.1 to 1.0 micrometers.

[0012] A more recent advance in particle coating technology is the useof fluidized bed systems, and in particular, magnetic fluidized bedsystems such as that shows in U.S. Pat. No. 5,962,082 (Hendrickson etal.). There, a magnetic field fluidizes a bed of magnetically responsiveparticles. The magnetically responsive particles and/or other particlescarried into a fluidized bed are coated with a material (e.g., a liquid)provided in the fluidized environment. The coating composition may evenbe transferred from the magnetic particles to non-magnetic particles.This process provides excellent control over the coating thickness, canproduce large volumes of coated particles, and provides many otheradvantages.

[0013] U.S. Pat. No. 5,958,329 describes a method and apparatus forproducing nanoparticles (there defined as from 1 to 50 nano-meterdiameter particles) at a high rate. Two chambers are separated by anarrow duct. A source material is provided from a lower chamber wherethe source material is heated (e.g., to vaporization and thencontinuously fed into an upper chamber. In the upper chamber,nanoparticles are nucleated, the nanoparticles being formed when thevapor fed from the lower chamber collides with a gas (inert or reactive)in the upper chamber. A cooled deposit site (e.g., defined as finger107) collects the particles, which are then scraped from the collectionsite. The particles are said to move to the collection site in a naturalconnective flow stream.

[0014] U.S. Pat. No. 5,128,081 describes a method of preferential phaseseparation of aluminum oxide nanocrystalline ceramic material. Thenanoparticles are collected on a cold surface (20). Following oxidationof the particles, a vacuum chamber (in which the particles were formed)is evacuated and the oxide particles are collected and consolidatedunder various atmospheric conditions, such as vacuum and selectivelywith oxygen and/or air.

[0015] The collection process in these particle manufacturing andparticle treating processes is cumbersome, inefficient, costly,time-consuming and damaging to the particles. For the collectionprocess, the chamber must be opened and particles scraped from thedeposition surface. This requires a long term shut down of the system.Scraping of particles from the deposition surface will fracture someparticles and leave others agglomerated. Scraping can also damage thedeposition surface. The small elongate finger deposition surface allowsfor the production and collection of only small amounts of materialslayering of collected particles reduces the efficiency of depositiononto the surface. Coating of the particles can be done, but only asre-dispersion of the dried and agglomerated particles.

[0016] An alternative method of particle collection is filtration. Thisis performed by placing in sequence a source of particles, a filtrationmedium and a vacuum source. The filter has two surfaces, one frontsurface facing the particle source and the other rear surface facing thevacuum source. The reduced pressure at the rear surface allows thehigher pressure at the font surface to push gas and particles againstthe filter where the particles are entrapped. There are a number ofproblems in a filtration system, particularly when it is used withnanoparticles. For example, to collect nanoparticles having an averageparticles diameter of from 1 to 100 nanometers, the largest pore size inthe filter must be less than about 1 nanometer. It is difficult tomaintain an effective pressure across that filtration surface, evenbefore particles start collecting. As nanoparticles collect on thefilter surface, gas flow (and pressure driven movement) become morerestricted, fewer particles can collect, and process efficiencydiminishes. The particles clog pores rapidly and particles do notcollect efficiently.

[0017] U.S. Pat. No. 5,857,840 describes a vacuum pump system for makinga closed container vacuous, comprising a vacuum pump and a dustcollector provided on a pipe connecting the closed container and thevacuum pump, the pipe including:

[0018] a main pipe having a first main pipe which connects the closedcontainer and the collector and a second main pipe which connects thecentrifugal collector and the vacuum pump;

[0019] a bifurcated pipe which is branched out from the first main pipeand connected to the vacuum pump;

[0020] a metal mesh dust collector disposed on the bifurcated pipe; and

[0021] pipe switching means for switching over between the main pipe anda bifurcated pipe.

[0022] The dust collector is provided intermediate the source of dustand vacuum pump, which may include a dry pump.

SUMMARY OF THE INVENTION

[0023] A particle collection system with increased collection efficiencyfor the collection of nanoparticles comprises a source of particles, adry pumping system, and a particle collection surface. The position of adry pumping system in advance of the particle collection surfacemaintains a particle moving effort, without wetting particles andcausing them to agglomerate, and increases collection efficiency.

[0024] The placement of the collection units between the nanoparticlesource and vacuum pumps causes severe problems in maintaining systemvacuum and related high evaporation rates. Wet collection systems aredifficult to operate in a vacuum environment; however, the operation ofwet collection systems provides slurries in a number of differentsolvents, which can be post-treated by in-situ polymerization techniquesto coat the nanoparticles. The particles in the resulting slurries canbe coated with fluoropolymers, such as teflon and polyvinylidenedifluoride (PVdF) by in-situ polymerization methods. This differs fromearler work by the use of high pressure reactor technology to provide ateflon or PVdF coating onto the particle. This is the first knownapplication of these polymers in an in-situ polymer coating process.

[0025] A source of nanoparticles is provided. The source may be aprimary source where particles are being manufactured (e.g., sputtering,spray drying, aerial condensation, aerial polymerization, and the like).The source of nanoparticles may also be a secondary source of particles,where the particles have been previously manufactured and are beingseparately treated (e.g., coating, surface oxidation, surface etching,and the like). These nanoparticles are provided in a gaseous medium thatis of a sufficient gas density to be able to support the particles inflow. That is, there must be sufficient gas that when the gas is moved,the particles will be carried. With nanoparticles (Particles havingnumber average diameters of 1 to 100 nm, preferably 1 to 80 nm, or 1 to70 nm, and as low as 1 to 50 nm) only a small gas pressure is needed,such as at least 0.25 Torr although higher pressures greater than 0.25Torr, greater than 0.4 Torr, greater than 0.6 Torr, and greater than0.75 Torr greater than 0.9 Torr are preferred.

[0026] The gas-carrying medium may be or have been reactive with theparticles or may have some residual reactive materials in the gas. It ispreferred that the gas is relatively inert to the apparatus environment.Gases such as nitrogen, carbon dioxide, air and the like are preferred.

[0027] The propulsion system for the gas carrying medium and thenanoparticles is a dry mechanical pumping system for gases. A drypumping system is used to prevent contamination of the particles bylubricants. These dry pumping systems for gases are well known in thesemiconductor industry for conveying air, particulate and vapors withoutcollection occurring in the pump. They are pumping systems that utilizeoil-less seals to maintain vacuum conditions at the pump inlet. Examplesof such dry pumps and dry vacuum pumps in the literature are found inU.S. Pat. No. 4,452,572 (Robert Evrard) generates a dry vacuum whenacting as an additional stage to a conventional vacuum pump. It cites atubular diaphragm that admits a pressure differential across thediaphragm to allow the diaphragm to conform to the contour of thepumping chamber body and thus expel gas via a top valve. U.S. Pat. No.5,971,711 describes a control system for pumps, including dry pumpsbased on a Roots system pump.

[0028] U.S. Pat. No. 6,050,787 provides a dry pump comprising amagnetically responsive elastic tube stretched onto, thereby sealing to,a shaft with inlet and outlet ports at or adjacent to it's ends of thetube. Local to the inlet port a magnetic field is generated in theenclosing body. This field is substantially concentric to the tube,which then responds by expanding circumferentially towards the magneticfield. This creates a volume between the tube and shaft, the length oftube outside the influence of the magnetic field remains sealed upon theshaft. Subsequent movement of the magnetic field along the axis of thepump gives transport of this volume and any media now enclosed within itfrom the inlet port to the outlet port, whereupon reduction of themagnetic field results in exhaustion of the volume. This cycle resultsin pumping action.

[0029] Other general disclosures of mechanical dry pumps are provided inU.S. Pat. Nos. 6,090,222; 6,161,575; 5,846,062 (which describes a screwtype dry vacuum pump having dual shafts is disclosed, whereby theprocess gas is transported through three compartments, a gas admittancepump section, a central drive motor section, and a gas discharge pumpsection. By placing the drive motor in the center of the pump, itbecomes possible to design a pump having the dual shafts supported onlyat one end, thus enabling to mount the rotors at the free ends of thepump which are closed with end plates which can be removed easily forservicing the pump sections. Synchronous operation of the dual shaftpump by magnetic coupling enables to lower power consumption and toextend the range of operable pressures.

[0030] The collecting medium for the nanoparticles may compriseelectrostatic surface collectors, electrostatic filter collectors,porous surfaces (e.g., fused particle surfaces), centrifugal collectors,wet scrubbers, liquid media collectors and physical filter collectors.The liquid media collectors)with subsequent separation of the liquid andthe particulates) are more amenable in the practice of the presentinvention. Also know as wet scrubbers, these liquid collection media aremore amenable to this arrangement due to process and safety factorsallowing more volatile solvents to be utilized away from the formationchamber for the nanoparticles. Wet scrubbers also provide slurriessuitable for post-treatment and polymer coating by in-situpolymerization, particularly in the case of fluoropolymer coatings.Examples of this are Teflon, Polyvinylidene difluoride (PVdF), and theirrespective copolymers.

[0031] The use of the present arrangement of nanoparticle source, drypump and collector has been found to increase particle collectionefficiency by as much as 100% in comparison to the conventional source,filter pump system, even where the same nanoparticle source is present,the same filter and the same pump is used in the different order. Theutilization of this arrangement of the pumping scheme may also benefitthe collection of the nanoparticles. By injecting low volatilitysolvents into the inlet of the pump with the nanoparticle loaded gasstream, the dry pump may also be utilized as a wet scrubber with betterthan 90% collection efficiency. Suitable solvents are the variousavailable Isopar® media and Purasolv® media.

SUMMARY OF THE INVENTION

[0032] Small particles of metals are prepared by an evaporative methodwith a unique collection method that increases the production efficiencyof the process by dramatic degrees. The process comprises evaporating ametal and then providing a mechanical pump that either draws the gasphase metal into a liquid condensation-collection zone or combines aliquid condensation-collection zone within the mechanical pump. Thenon-metal gaseous material remaining after condensation removal of themetal material is withdrawn from the material stream, while the liquidcondensing phase with the condensed metal particles is separated, theliquid condensing phase carrier removed, and the particles collected. Ascompared to known prior art methods, the use of the intermediatepositioned mechanical pump or contemporaneous mechanical pump andcondensation-collection zone increases the overallcollection/manufacturing efficiency of the process by at least 25%.

BRIEF DESCRIPTION OF THE FIGURES

[0033]FIG. 1 is a schematic diagram of one embodiment of an apparatusfor providing metallic nanoparticle dispersions of the presentinvention.

[0034]FIGS. 2a and 2 b show various crucible designs that have beenimproved upon in the practice of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0035] The existence of nanoparticulate materials such as metals,organic materials, metal oxides and other pigments has been known forseveral years now; however, the production of these materials is stillextremely low from the existing processes. This has had a detrimentaleffect on the availability and therefore the utilization of thesematerials in various products. Several applications of this materialhave remained unattained due to the lack of suitable large scalesupplies of this material to incorporate into the end products or atleast proof out these materials in research and development work.

[0036] The most frequently used technique to form nanophase materials,such as metals, is the inert gas condensation, or dynamic gascondensation, method (Siegel, R. W. and Eastman, J. A., MaterialResearch Symposium Proceedings, 132, p. 3, 1989; and Granquist, C. G.and Buhrman, J., J. Appl. Phys., 47, p. 2200, 1976). In this technique,a metal is vaporized and recondensed by contact with a low pressure fluxof inert gas. The typical method used to melt and vaporize the metal tobe converted into nanoparticles has been resistive heating. Through theuse of either a tungsten or tantalum heating element or an intermetallicceramic bar, metal is evaporated from conductive heating by contactingthe hot surfaces of the material. The use of intermetallic ceramics isfavored over the metallic heating elements due to the ability of somemetals to corrode some other metals by an alloying process. This causesshorting of the resistive circuit by overamping, etc. This has resultedin the use of the intermetallic materials (AlN, BN and TiB₂) in aluminumevaporation, as an example

[0037] A resistively heated bar can reach temperatures of 1500-1600° C.in a high vacuum (<10⁻⁵ Torr). At these temperatures, pressures of ˜5Torr or less are needed to “flash” evaporate most metals.“Flash”evaporation is that condition where the molten metal is superheatedbeyond the boiling point of the metal at certain conditions and isinstantly converted to vapor (Learn, A. J., Thin Solid Films, 20, p.261, 1974). In a high vacuum system, it is relatively easy to supportboth the melting and vaporization of the metal if the appropriateamounts of energy are available. For the resistive heating method;however, there is only enough energy available to vaporize small amountsof material at one time. This is often why wire feed mechanisms arecommonly used with resistive heating/vaporization methods (Rynee, D. M.,Solid State Tech., 11, p. 48, 1968; Learn, A. J., J. Electrochem. Soc.,123(6), p. 894, 1976). As a conductive method of heating andvaporization, the energy transferred by conduction in the resistiveheating methods is maximized by use of a small contact area thatcontinually evaporates small amounts of material supplied by the wirefeed mechanism. The wire feed mechanism is uniquely suited to therequirements of the resistive heating/vaporization techniques, and theevaporation/production rates are then determined by the speed of thewire feed mechanism matching, but not exceeding the amount of metal thatcould be evaporated by the conduction-driven methods. Even at highvoltages and feed rates; however, the end production rate is not anindustrially suitable method for the manufacture of metallicnanoparticles. A typical resistive bar operates at 4 volts and 830 ampsand dissipates a power of 3324 watts. The temperature generated is˜1500-1600° C., as noted previously. The typical evaporation rate for ametal such as aluminum is 0.10 grams/min per bar. This is quite low andbatteries of resistive bars are often used to form an aggregateproduction rate suitable for vapor coating. This is sufficient for vaporcoating substrates to a depth of less than two-tenths of a micron as istypically done for Mylar polyester and nylon web coatings in commerce.Although this would suffice for a vapor coating operation, it is not ascaleable procedure for large scale nanoparticle production.

[0038] Resistive heating also has other drawbacks as well. It has beenobserved that a temperature gradient appeared in the resistively heatedbar from convection when exposed to the inert gas needed to nucleate thevapor into the nanoparticles. The ends of the bar would be hotter thanthe center where the metal would be fed onto the bar. This has twooutcomes. First, the excess energy needed to vaporize the metal is lostdue to convective heating of the inert gas stream. Second, thetemperature of the bar also drops considerably due to the convectivelosses. This drop in temperature puts the overall operating parametersof the system (temperature, pressure) into undesirable areas. Inrelative comparative terms, undesirable effects correspond to the slowevaporation of water below its boiling point versus the rapidevaporation and steam evolution that occurs when the water issuperheated beyond its boiling point and the water is converted directlyinto steam.

[0039] The additional pressure that occurs from the introduction of agas stream into the system is also a factor. Most rough vacuum pumps canreach ultimate pressures of less than 50 mTorr in a closed vacuum systemwithout the introduction of gas into the system. The addition of gasflow to the vacuum chamber changes this base pressure considerably asthe expansion of the ambient condition gas at near vacuum yields ahigher gas volume that must be pumped from the system. This basepressure will also increase with the presence of line expansions andconstrictions that occur with the presence of vacuum chambers and trapsin the system (Brunner, W. F. and Batzer, T. H., “Practical VacuumTechniques”, Krieger Publishing Co., New York, 1974; and O'Hanlon, J.F., “A User's Guide to Vacuum Technology”, Wiley, N.Y., 1980. Thisincrease in base pressure coupled with the temperature drops observedwith gas contact on the resistively heated bar puts the operatingparameters of the system below the vapor pressure curve.

[0040] For this inert gas condensation process to work at a reasonableproduction rate, a method of vaporizing the metal, in this case,aluminum, must be found that allows a high operating temperature for thesystem while maintaining the proper gas flow and pressurecharacteristics in the system.

[0041] A high vaporization rate of material may be effected by inductiveheating. With the ability to couple directly into the metal itself toheat and vaporize it, it is an obvious technique to utilize in a largescale production method. Due to the ability to input the energyavailable directly into the metal itself, there may also be a largeroperating window in terms of temperature and pressure. A possible smallscale induction unit that may be used is a Mark 6, 10 kHz, 15 kW Pillarunit. Due to coil and line losses, only 80% of the 15 kW is available(12 kW) for introduction into the metal charge in the inductioncrucible. With the antiferrous metals, only 30 to 50% of this inductionunit power can couple effectively with the metal charge to heat andvaporize the metal. For a ferrous metal, the total amount of power canbe coupled into the metal charge. The coupling of this technology withthe melting and vaporization of metals has been well established sinceWorld War II (Davies, E. J., and Simpson, P., “Induction HeatingHandbook”, McGraw-Hill, London, 1979; Davies, E. J., “Conduction andInduction Heating”, Peregrinus, London, 1990). The results from the 15kW unit employed can be scaled to a standard 600 kW unit or higher powerdepending on the custom design and manufacture available for thesesystems. Vacuum chambers and induction coils are readily available orcan be manufactured easily. This comprises the first part of thenon-public system developed during this project. The second part of thesystem is the vacuum pumping system, which has been well established anddeveloped from the vapor coating and semiconductor industries. The thirdpart of the system is the collection of the metallic nanoparticles andtheir dispersion into liquid media. The last part of the system is thefluoropolymer coating portion of the process where the nanoparticles arecoated with the protective polymer coating to prevent oxidation.

[0042] Collection of the metallic nanoparticles is also a problem in theproduction process. Most previous attempts for the production ofnanophase materials consisted of vaporizing the metal feedstocks atultralow vacuum conditions and collecting the nanoparticles formed on aliquid nitrogen cold finger system by thermophoresis or the walls of alarge volume expansion chamber by impingement and settling (Siegel, R.W. and Eastman, J. A., Material Research Symposium Proceedings, 132, p.3, 1989; Granquist, C. G. and Buhrman, J., J. Appl. Phys., 47, p. 2200,1976). This has several disadvantages in collection including theinability to form unique unagglomerated nanoparticles. Although meanparticle sizes of <10 nm are claimed, this is mainly the primaryparticle size of the crystallites of the material which are formed.These crystallites are agglomerated to particle sizes which are muchhigher than this mean crystallite size during the collection process.This collection method also leads to oxidation problems with pure metalsystems as the surfaces of the dry, reactive nanoparticles need to bepassivated in some manner before further handling. This is typicallydone by oxidizing the outer surface of the nanoparticles by thecontrolled admission of oxygen to the chamber to form a thin oxide layerto eliminate the possibility of their burning in uncontrolledatmospheres. This oxidation essentially destroys useful fuel in the bulkof the nanoparticle. Most passivation layers for metals are up to 10 nmin depth. For a 30 nm or lower diameter particle, this is most of themetal present. However, thinner layers of the passivation oxide havebeen achieved with difficulty (Granquist, C. G. and Buhrman, J., J.Appl. Phys., 47, p. 2200, 1976; Aumann, C. E., Skofronick, G. L. andMartin, J. A., J. Vac. Sci. Tech. B, 13(3), p. 1178, 1995. Dixon, J. P.,Martin, J. A., and Thompson, D., U.S. Pat. No. 5,717,159, (February1997).

[0043] Collection in liquids yields two advantages. First, it protectsthe surface of the particles from oxidation by providing a temporaryliquid cover over them. Second, the process provides a slurry that canbe handled in a safe fashion. The liquid dispersion medium can be asolvent, polymer monomer, or prepolymers (Dixon, J. P., Martin, J. A.,and Thompson, D., U.S. Pat. No. 5,717,159, (February 1997); Hendrickson,W. A., Wright, R. E., Allen, R. C., Baker, J. A., and Lamanna, W. M.,U.S. Pat. No. 5,030,669.

[0044] Previous work with evaporated pigments has found that theimmediate dispersion of pigment nanoparticles is also beneficial in theformation of a stable dispersion of nanoparticles in the collectionliquid. The collection systems previously utilized were sparge unitsthat bubbled the dust-laden gas through the collection liquid andscrubbed the nanoparticle materials from the gas itself.

[0045] Fluoropolymer coatings to prevent oxidation of the metallicnanoparticles also can be applied. The application of these coatings canbe done by an in-situ growth of the coatings on the metallicnanoparticles in the non-aqueous slurries formed. This is similar towork where polystyrene, polyaniline and other coatings were applied toinorganic oxides. The use of a fluoropolymer was to provide a pliable,noncracking coating to the outside of the reactive metallicnanoparticle. This formation of fluoropolymers and their copolymers hasbeen done for several years in the production of Viton and PVdF. Incontrast to the former particle coating techniques in the prior art,these polymerization reactions need to be run in high pressure reactorsto liquefy the gaseous fluoromonomers and allow the polymerizations toproceed at a reasonable rate. However, this technology is welldeveloped, and standard equipment and parts are available for thisprocess. Fluoropolymers, such as polyvinylidene fluoride (PVdF) and itscopolymer Viton®, have been produced commercially on an industrial scalesince the early 1960's (Rexford, D. R., U.S. Pat. No. 3,051,677; and Lo,E. S., U.S. Pat. No. 3,178,399.

[0046] The initial system set-up for the invention that was utilizedhere is shown in FIG. 1. Greater gas flow rates to nucleate the metalvapor formed are found to be desirable near the increasing ratesapproachable with induction heating, and these can be provided mostefficiently according to the practice of the present invention. Althoughhigh amounts of vapor were formed with low gas flow rates and theresulting low pressures, it was often not being converted intonanoparticles that could be collected later.

[0047] The amount of metal spatter occurring during the process waseliminated by utilizing the crucible design shown in FIG. 2 to enhancethe coupling of the inductive field with the molten metal whilecontaining the spatter that is occurring. Increasing the amount ofnanoparticles produced to almost a 100% conversion from the vapor phasewas achieved by increasing the amount of inert gas, either nitrogen orargon, flowing through the ceramic guide tube and going around thecrucible during vapor formation. As the wall thickness of the crucibleincreased to reduce metal spatter, the amount of material that could becharged to the crucible also decreased and would require a constantrecharging of the molten metal during the evaporation process. A wirefeed mechanism can be installed that will allow this process to occur.The typical collection liquid for the aluminum nanoparticles had beenIsopar®G, an aliphatic hydrocarbon utilized in liquid toner work whichhad been employed in earlier research on this process (Matijevic, E.,Chem. Mat., 5, p. 412, 1993; Johnson, J. E. and Matijevic, E., Coll.Poly. Sci., 270, p. 353, 1992; Huang, C., Partch, R. E., and Matijevic,E., J. Coll. Int. Sci., 170, p. 275, 1995; Huang, C. and Matijevic, E.,J. Mat. Res., 10(5), p. 1329, 1995; Partch, R. E., Gangolli, S. G.,Matijevic, E., Cai, W., and Arajs, S., J. Coll. Int. Sci., 144, p. 27,1991).

[0048] Utilization of the prior art for the collection of nanoparticlesprior to the vacuum source yielded low production rates (0.5 gm/min orless). Although there was sufficient energy available from the 15 kWPillar unit to vaporize nearly 2-3 lbs of aluminum per hour and thetemperature was adequate (˜1500-1600° C.), only a minor portion of thisenergy was actually expended in the vaporization of the metal usingprior art processes due to the high system pressure resulting from thepressure drop across the collection unit. A new method for utilizing thepower from the induction unit was needed and this required the deepeningof the system vacuum in order to achieve operating parameters near orabove the vapor pressure curve shown in FIG. 1.

[0049] A substantially improved vacuum system was designed according toFIG. 1 that deepened the system vacuum to values appropriate to flashevaporate metals utilizing the induction unit power while eitherconveying the nanoparticles through the vacuum source or collecting themafter the pump. By eliminating the pressure drops occurring due to thepresence of the collection unit,the evaporation rates of the system wereincreased from 0.5 gm/min to ˜2 gram/minute and then to 10 g/minute foraluminum as the crucible design was changed to that in FIG. 2. Althoughthe evaporation rate and corresponding nanopowder production rate wereincreased substantially, the ability to collect the nanoparticlematerials by sparging the dust-laden gas through a collection liquid waslimited severely. To achieve the higher production rate throughdeepening the system vacuum, a higher gas flow also occurred, whichincreased the amount of gas sparged through the collection liquid by afactor of four. With the lower flow rate (˜10 liter/min) at the lowerproduction rates, the collection of the nanopowder was slightly lessthan 50% using the sparge collection vessels of the prior art. With thehigher flow rate of gas through the sparge vessels, the collection ofthe nanopowder dropped severely and the entrainment of the slurry in thegas stream also became a problem.

[0050] An improved method of liquid collection was needed in order tosafely handle and treat the nanoparticulate material in a consistentmanner. Prior art work arrived at the use of high power aspirators andventuris to both collect the nanoparticles produced in liquids and alsoto supply vacuum to the evaporation chamber at the same time. Althoughit worked sufficiently well at the lab scale, it was difficult toimplement at the pilot plant scale for materials other than pigments dueto the amount of vacuum supplied by these devices. With the use of thecombined liquid collection/vacuum supply system with dry mechanicalpumps employed to convey or collect the nanoparticles as shown in FIG. 1and as previously detailed, many of the prior obstacles to high rateevaporation and liquid collection were overcome. This current capabilityof the system is an evaporation rate of 10 gm/minute for aluminum as themetal with a liquid collection efficiency of nearly 90% of the materialcontacting the scrubbing system.

[0051] The evaporation rate of 10 gm/min for the aluminum metalindicates that there is considerable energy loss in the system and thatonly 25-33% of the total power of the system is being used to vaporizethe material. These losses may be occurring in heating the crucible,heating the cold metal to its melting and boiling points or in generalconvective/radiation heat losses to the gas stream in the system.

[0052] The large scale production of material from this system also hasconsequences in terms of particle size control and materials corrosion.There are often references in the literature about the ability to tailorthe particle size of nanoparticles formed by the inert gas condensationmethod by increasing the back pressure of inert flux gas in the system(Siegel, R. W. and Eastman, J. A., Material Research SymposiumProceedings, 132, p. 3, 1989; Granquist, C. G. and Buhrman, J., J. Appl.Phys., 47, p. 2200, 1976; Aumann, C. E., Skofronick, G. L. and Martin,J. A., J. Vac. Sci. Tech. B, 13(3), p. 1178, 1995). This pumping scheme,as shown in FIG. 1 is more amenable to these techniques of than are thesystems of the prior art.

[0053] A full-scale system would have four major pieces of equipment toproduce either a solvent or solvent/prepolymer nanoparticle slurry. Thefour main pieces of equipment would be 1) the induction power source, 2)the vacuum chamber and feed systems, 3) the vacuum pumps and 4) theliquid collection system. At the highest rates of production possible, a15 kW unit can evaporate one pound per hour with a 90% collectionefficiency or greater. This translates into a little over 30 lbs/hour(66 kg/hr) for a 600 kW unit. This is a rate that is comparable to thatobtainable from other industrial systems in the field, such as plasmaand flame combustion systems. It would yield one-quarter of a millionpounds per year of nanophase material for one unit. Although this is notthe tons/hour production levels available from flame combustion units,combining the output from several units into one production batterywould achieve outputs of material comparable to this. From a safetystandpoint, it may also be sensible to have a number of smaller unitsoperating rather than one extremely large unit custom-designed unit. Ifan incident did occur, only one small unit would undergo a catastrophicfailure, reducing property loss and personal injury. The loss of onesmall unit would also enable production to continue for the entirefacility while the faulty unit was repaired rather than a totalproduction shutdown.

[0054] In addition to military applications, this fully developed systemfor the production of nanoparticles, particularly pigment or metalnanoparticles and especially aluminum nanoparticles and theirdispersions will find significant outlets in forming pigment dispersionsfor paints, toners, inks, colorant systems, plastic/resin coloration,coating colorization, explosives, munitions, fuel additives,pharmaceutical coloration, and the like.

[0055] The production and efficiency of collection rate ofnanoparticulate materials has been increased significantly to rates thatare scaleable to large sized production lots by practice of theinvention. The utilization of nanophase metal particles in particulartechnical areas may also be dependent, in part, on the dispersion of thenanoparticles into particular liquid media of choice. An added bonus ofthe program has been the improved ability to collect and disperse thenanoparticles into different liquid media. This liquid media can besolvents, carriers, reactive compositions, coating solutions, oils,polymer monomers or prepolymers or mixtures of these liquids. Thisfeature, in addition to the increased collection/condensation efficiencyis an added advantage of the inventive process over other processespresently available in this field. The dispersion of these nanoparticlesinto these different liquids aids in their ease of processing and alsoin the protection of any reactive surfaces from oxidation and evenphysical damage.

[0056] The technology that has been developed by the inventors also hasapplications outside the narrow confines of any specific metal or alloy,but is generically useful for any metal or alloy that can be provided ina vapor state and which can be condensed by cooling in a liquid medium.The particles can also be collected wet in different liquid media inwhich they can form stable dispersions for use in a wide variety oftechnical areas such as discussed above.

[0057] In this application:

[0058] “ultrafine” means having a mean number average diameter of lessthan 0.1 micrometer, preferably in the range of 0.001 to 0.1 micrometer,more preferably in the range of 0.001 to 0.08 micrometer, mostpreferably in the range of 0.001 to 0.05 micrometer; and having astandard deviation in the range of .+−.0.5 x, where x is the mean numberaverage particle diameter;

[0059] “gas evaporation technique” means any method involving theevaporation of a metal, metals or alloys in the presence of anon-reactive gas to provide ultrafine metal or alloy particulate.

[0060] The present invention is capable of providing an aqueous ornonaqueous metal or alloy dispersion comprising metal or alloy particlesor an aqueous dispersion of these particles, the particles having ameans number average particle diameter in the range of 0.001 to 0.1micrometer (1 to 100 nanometers), preferably dispersed in a dispersingmedium. Preferably the mean number average particle diameter is in therange 0.001 to 0.08 micrometer (1 to 80 nanometers) and most preferably0.001 to 0.05 micrometer (1 to 50 nanometers). The dispersions cancontain pigment from 0.001 to 50% by weight, preferably from 0.001 to25% by weight, and more preferably, from 0.001 to 10% by weight of thetotal composition. Narrow size distribution ranges of less than ±25% or±15% of the average diameter are also able to be formed and aredesirable.

[0061] A non-limiting example of the manner in which a dispersionaccording to the present invention may be prepared includes:

[0062] a) vaporizing a metal, metals or alloy in the presence of anon-reactive gas stream (or introducing the vaporized metal into anon-reactive gas stream) or a reactive gas stream to provide ultrafineparticles (especially metal and alloy particles),

[0063] b) transporting the ultrafine particles suspended in the gasstream by a mechanical pump to a liquid dispersing medium, as forexample, a mechanical pump located before the dispersing medium orcontaining the liquid dispersing medium, the gas containing theparticles being forced into the liquid dispersing medium or the gascontaining the particles intimately contacting the dispersing medium, toprovide a dispersion of particles in the medium (with vaporized metalcondensing in the liquid medium),

[0064] c) the gas (absent the particles) is then separated from thedispersing medium (e.g., by bleeding out the gas, allowing the gas torise to an exit area within a chamber, etc.), and

[0065] d) the dispersing medium is then used to carry the collectedmetal or alloy particles as a dispersion or the dispersing medium isthen optionally being separated from the pigment particles to providenon-dispersed metal or alloy particles.

[0066] Metals having a vaporization temperature below 3000° C. are wellknown in the art, and include, for example, Li, Na, K, Rb, Cs, Fr, Be,Mg, Ca, Sr, Ba, Ra, Y, La, Ac, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc,Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, In, Tl,Sn, Pb, mixtures and alloys of these metals and even the lanthanides andactinides, if desired.

[0067] Several methods are available for characterizing a particledispersion. The most common involves the particle size distributionexpressed as the weight percentage of particle falling within a givensize range. Typical size limits for metal particles desired in thepractice of this invention are about 0.01 to 1.00 micrometer (10 to 1000nm).

[0068] These values are indicative of the overall range of particlesizes typically encountered after conventional dispersion techniques.The distribution of particle sizes is dependent on the means of particleformation. Where mechanical milling is used to comminute the particles,extremely wide distributions result and the morphology of metalparticulates often change significantly.

[0069] In the present invention, the vapor phase of evaporated particlesand the particles themselves may be generated by any evaporative processsuch as subliming or any other evaporation process for metals atsubatmospheric atmospheric or superatmospheric pressures in the presenceof a non-reactive gas to generate ultrafine metal or alloy particles andthen effecting direct introduction into a dispersing medium, such asdescribed herein, has not been taught.

[0070] Where the term “metal” is used herein, it is intended to includemetals, mixtures of metals and alloys.

[0071] Dispersing media useful in the present invention include anyliquid, aqueous (where the metal does not rapidly react with water atcollection conditions) or nonaqueous (for most metals). Fluids having aviscosity up to 100,000 P or more are envisioned as useful. Preferredviscosities are less than 5000 cP, more preferably less than 3000 cP,and most preferably less than 1000 cP. Representative dispersing mediainclude water, gelatin/water emulsion, 10 alcohol/water, includingmixtures such as ethanol/water, glycerol/water, etc. and polar organicliquids such as acetone, 2-butanone, cyclohexanone, 2-undecanone,methanol, ethanol, isopropanol, glycerol, ethylene glycol, ethylacetate, alkanes (e.g., hexane, cyclohexane), methyl methacrylate,2-hydroxyethylmethacrylate, chloroform, methylene chloride,alkylalkanolamines, such as 2-dimethylaminoethanol,1-dimethylamino-2-propanol, 1-diethylamino-2-propanol,2-dimethylamino-2-methyl-1-propanol, and 2-dibutylamino ethanol, andcombinations thereof.

[0072] Useful nonpolar organic liquids include hexane, a mixture ofisoparaffinic hydrocarbons, b.p. 156° C.-176° C. (Isopar G®, Exxon,Houston, Tex.), benzene, toluene, xylenes, styrene, alkylbenzenes, andcombinations thereof. In addition, liquid polymers such aspolydimethylsiloxane (e.g., DC200™ MW_(n)=200, Dow Chemical, Midland,Mich.), polydimethyl-co-methylphenylsiloxane (e.g., DC ₇₀₄™, DowChemical), polyethylene glycol (e.g. Carbowax® 200, Carbowax® 400, andCarbowax® 600, MW_(n)=200, 400, and 600, respectively, Union CarbideCorp., Danbury, Conn.), a polymer comprising perfluoropolyether segments(LTM™, 3M, St. Paul, Minn.), and polycaprolactones (Placcel™ 305, 303,308, MW_(n) =300-850, Daicel Chemical Ind. Co. Ltd., Tokyo, Japan) maybe used.

[0073] Additionally, external heat may be applied to melt a solid (e.g.,a polymer, a wax, or any low melting organic compound such asnaphthalene) and generate a liquid dispersing medium suitable for use inthe present invention. Examples of solids that may be used includeparaffin wax, low molecular weight polyester (e.g., FA™-300, EastmanChemical Co., Rochester, N.Y.), and polyethylene.

[0074] The dispersing medium may be a pure liquid or a mixture ofliquids and may contain additional ingredients, including inorganic andorganic soluble materials and mixtures thereof. Such additives includesurface-active agents, soluble polymers, insoluble particulates, acids,bases, and salts.

[0075] By surface active agent is meant an additive that has a preferredspatial orientation at an interface, e.g. large molecules having ahydrophilic head group and a hydrophobic tail (e.g. OLOA™ 1200, ChevronCorp., Richfield, Calif., and Amoco™ 9250, Amoco Chemical Co.,Naperville, Ill.). The weight percent of surface active agent todispersing medium can be from 0 to 20%, preferably 0 to 10%, and morepreferably 0 to 5%. Other surface active agents useful in the presentinvention are well known to those skilled in the art.

[0076] Soluble polymers useful as additives in the present invention,for example, in the manufacture of pigmented films, include polystyrene,polystyrene-co-butadiene, poly(methyl methacrylate), poly(ethylmethacrylate), poly(butyl acrylate), poly(4-vinylpyridine),poly(2-vinylpyridine), poly(vinylpyrollidone), poly(2-hydroxyethylmethacrylate), poly(ethylene terephthalate),polystyrene-co-4-vinylpyridine, polystyrene-co-2-vinylpyridine,polyethyleneglycol, poly(ethylene oxide), poly(propylene oxide),polyethylene, polypropylene, poly(acrylonitrile), poly(phenyl vinylenecarbonate), poly(vinyl acetate), poly(vinyl alcohol), poly(vinyltrifluoroacetate), poly(vinyl chloride), poly(ethylene-co-propyleneadipate), poly(1,4-phenylene sebacate), poly(3,5-dimethyl-1,4-phenylenesulfonate), poly (.beta.-alanine), poly(hexamethylenesebacamide),poly(vinyl cymantrene-co-4-vinylpyridine), etc. The percent of solublepolymer in the dispersing medium may be from 0 to 70% by weight,preferably 0 to 50%, more preferably 0 to 30%, and most preferably 0 to25%, or each range with at least 0.5% minimum therein. Other polymersuseful in the present invention are known to those skilled in the art.

[0077] Insoluble particulates useful as additives in the dispersingmedium of the present invention, for example, in the manufacture ofpigmented composite structures, include latex particles, kaolin,alumina, glass microspheres, and other common fillers known to thoseskilled in the art. The weight percent of filler compared to the totaldispersion can be from 0 to 80%, preferably 0 to 60%, and morepreferably 0 to 50%. The high specific heat additives may assist inmoderating the temperature of the dispersing medium.

[0078] The non-reactive gas can be virtually any gas that does not reactwith the metal under the conditions of the experiment. Typical choicesare He, Ne, Ar, Xe, and N₂. Mixtures of two or more non-reactive gasescan also be used. The non-reactive gases generally are at roomtemperature, but the temperature can be elevated or reduced as desired.The term reactive includes 1) direct reaction with the particles, as inthe case of metals, for example, with O₂, NO, NO₂, CO₂, CO, AsH₃, H₂S,H₂Se, NH₃, trimethylchlorosilane, methylamine, ethylene oxide, water,HF, HCl, or SO₂, or combinations thereof, to form the correspondingoxides or other compounds; 2) wetting, as described in UK Patent 736,590to increase dispersibility in which particles are exposed to the vaporof a volatile liquid which may be identical to the dispersing medium ormay be miscible with the dispersing medium, prior to contacting the bulkdispersing medium so as to create a solid/liquid interface while theparticles are suspended in the gas stream; and 3) adsorption, in which avolatile substance is introduced in the gas prior to contacting thedispersing medium, similar to wetting, but the substance is either not aliquid under normal conditions (atmospheric pressure and 25° C.), thesubstance is not miscible with the dispersing medium, or else thesubstance acts to protect the surface of the ultrafine metal particlesfrom the dispersing medium or additives within the dispersing medium.Typical substances that could be adsorbed include polymers such aspoly(methylmethacrylate) and polystyrene, and surface active agents.

[0079] Temperatures for evaporation of metals useful in the method ofthe present invention depend on the type of materials being used andgenerally range from 25° C. to around 500° C. when organic pigments areused and from 25° C. to around 1200° C. or even 25° C. to 3000° C.

[0080] Temperatures of the dispersing medium useful in the method of thepresent invention depend on the particular medium being used andgenerally range from −78° C. to 400° C., preferably from −50° C. to 300°C., and most preferably from 0° C. to 200° C.

[0081] Pressures useful in the method of the present invention rangefrom about 0.001 to 300 torr, preferably 0.01 to 200 torr, morepreferably from 0.01 to 100 torr, and most preferably from 0.1 to 50torr. The composition of the combination non-reactive and reactive gasstream can be from about 5 to 100% non-reactive gas or combination ofnon-reactive gases, preferably from 25 to 100%, more preferably from 50to 100%.

[0082] An apparatus for providing dispersions of ultrafine metalparticles comprises:

[0083] a) a furnace connected to a collection vessel, the furnacecontaining a heating means (e.g., resistive, inductive, e-beam,infrared, laser, plasma jet) and adapted to contain at least a first andoptionally a second gas inlet tube, said second tube being locateddownstream from said first tube, and a mechanical pump for evacuatingthe furnace and directing the gas phase carrying evaporated metalparticle to the collection zone or vessel, the zone and/or vesselcontaining a dispersing medium;

[0084] b) an optional system (e.g., a ceramic, plastic, or metalcrucible or slab that can be preloaded with material or which can becontinuously or batch-wise fed during operation of the apparatus, or theelectrodes can be the means) for introducing a metal into the furnaceand evacuation thereof;

[0085] c) optionally a system (e.g., a micro metering valve, electronicflow controller, or gas dispersing tube) for introducing through thefirst inlet tube a first, non-reactive gas stream into the furnace;

[0086] e) an evaporating or gas phase producing system (e.g., energyinput as by e-beam, infrared, laser, inductive, resistive, or plasmajet) for evaporating of generating a gas phase of the metal particlesinto the first gas stream;

[0087] f) a collection/condensation medium between or coincident withthe evaporating or gas phase producing system for allowing condensationof the vaporated metal particles (e.g., decreasing the temperature,raising the pressure, changing the chemical nature of the non-reactivegas, controlling the length of the transfer tube, controlling the gasflow rate, or combinations thereof) in the first gas stream to produce adispersion of ultrafine metal particles in the first gas stream in adispersing medium in the collection/condensation zone;

[0088] g) optionally, a system (e.g., tube, valve, pipe, a micrometering valve, electronic flow controller, or gas dispersing tube) forintroducing into the furnace through the second inlet tube a second,reactive gas stream, to allow reaction with the metal particles, toprovide ultrafine metal particles;

[0089] h) a region within the system for collecting particles in thecollection/condensation vessel (e.g., bubbling into or impingingparticles onto the dispersing medium).

[0090] The innovation described herein involves at least a repositioningof the vacuum pump in the system which a) allows a higher level ofvacuum to be achieved, b) reduces the particle size of the metalparticles formed and c) increases the efficiency of wet collection ofthe nanoparticles formed to as much as greater than 95%. This is asubstantial improvement over the prior art where the wet collection ofpigment particles occurred prior to the source of vacuum in the system.In prior art, the efficiency of the wet collection was a maximum of 50%at low gas flow rates (e.g., 2 liters/minute). This modest level ofefficiency drops substantially at higher gas flow rates through thesystem. The present invention can use higher flow rates, higher than 3liters/minute, higher than five liters/minute, higher than seven litersper minute, higher than 10 or 20 liters/minute and even higher than 50liters per minute and provide collection efficiency rates of greaterthan 80%, greater than 90% in some cases, and still as high as 95% insome other cases.

[0091] With the presence of nanoparticles in the gas stream, oil sealedmechanical pumps do not function in this altered processing scheme. Dry,mechanical pumps that utilize gas-purged bearings are the most preferredfor this application. These pumps can tolerate the presence of largeamounts of particulate in the gas streams that are being pumped andconvey the particulate from the inlet to the exhaust of the pump.Various models can also convey various liquids and vapors through theirinteriors. These pumps are in wide-spread usage in the semiconductorindustry. For this application, scroll pumps did not provide sufficientperformance without powder buildup in the interior of the pump. Dry lobeand screw pumps provided a sufficient amount of vacuum for theevaporation processes without powder build-up. Most preferred were dryscrew pumps that could tolerate the presence of low volatility liquids(Isopar®, Dowanal®, Purasolv®, etc.) in the pump mechanism. Theseliquids could be injected into the inlet of the vacuum pump and used asscrubbing/condensation/collection media for the nanoparticles formed.The collection efficiency of this method is >95% of the nanoparticulatematerial entering the vacuum pump. Higher volatility liquids and viscousliquids as the collection/dispersion/scrubbing media (e.g., prepolymers,polymers, monomers) required the use of an alternate wet collectionsystem, such as a venturi scrubber, positioned after the vacuum pump.These pumps typically operated at 1-10 Torr utilizing gas flows of up to50 liters/min of an inert gas at ambient or modified conditions.

[0092] With the presence of nanoparticles in the gas stream, oil sealedmechanical pumps do not function in this altered processing scheme. Dry,mechanical pumps which utilize gas-purged bearings are the mostpreferred for this application. These pumps can tolerate the presence oflarge amounts of particulate in the gas streams that are being pumpedand convey the particulate from the inlet to the exhaust of the pump.Various models can also convey various liquids and vapors through theirinteriors. These pumps are in wide-spread usage in the semiconductorindustry. For this application, scroll pumps did not provide sufficientperformance without powder buildup in the interior of the pump. Dry lobeand screw pumps provided a sufficient amount of vacuum for theevaporation processes without powder build-up. Most preferred were dryscrew pumps that could tolerate the presence of low volatility liquids(Isopar®, Dowanal®, Purasolv®, kerosene, diesel fuel, etc.) in the pumpmechanism. These liquids could be injected into the inlet of the vacuumpump and used to wash the nanoparticles formed out of the pump toprevent buildup and shutdown of the system.

[0093] In the case where inductive heating was used to evaporate themetal, the coils for a 15 kW, 10 kHz induction unit were placedvertically inside a 1.5 cubic foot vacuum chamber attached to a 170 scfm(standard cubic foot/minute) dry screw vacuum pump. The coils werepotted with an alumina insulation and either a boron nitride or aluminatube coated with boron nitride on the interior were used to channel thenanoparticle-loaded gas out of the chamber. Boron nitride was the mostpreferred coating or potting for this application. A crucible for metalevaporation was placed inside the coils and guide tube at an appropriateheight and placement. The crucible used was of graphite construction formetal evaporation was most preferred with materials that do not formcarbides with graphite (Cu, Ag, etc.) although ceramics materials (BN,BN—TiBr2, etc.) could be used as well. For metals which did formcarbides with graphite (Al, Si, etc.), a ceramic liner (boronnitride-titanium diboride, boron nitride-titanium diboride-aluminumnitride, or boron nitride) was most preferred with a boron nitridecoating or mechanical sleeve around the outside of the crucible toprevent carbide formation. Schematics of the two designs are shown inFIG. 2.

[0094] A wire feed mechanism replenished the crucible after each metalcharge had been evaporated and converted to nanoparticles by inert gascondensation. The dust-laden gas was then conveyed to and through thedry screw vacuum pump, where it was either scrubbed out by injection oflow volatility solvents at the inlet of the pump (i.e., within a pumpchamber and therefore coincident with entrance to the pump) or passedthrough the pump (and therefore after entering and passing through thepump) and scrubbed out by a wet collection unit behind it. The slurryformed could then be used in the intended final product or used forfurther treatment of the nanoparticles formed.

EXAMPLE 1 Aluminum Nanoparticle Collection in Isopar® G

[0095] The system as described above was used to evaporate aluminum wireand form nanoparticles from it. In this example, aluminum was used, butother metals have also been used, with mere adjustments in theevaporation temperature and the selection of the dispersant medium.Isopar®G was used as a collection fluid for the system and injected atrates of 0.25-0.5 liter/min into the inlet of the vacuum pump. Argon gasflow was maintained at a level of up to 20 liter/min to yield abackground pressure of ˜8 Torr in the chamber. The nanoparticles formedwere collected at 95% efficiency in the liquid slurry at a primaryparticle size of 0.03 microns.

EXAMPLE 2 Copper Nanoparticle Collection in Purasolv® BL

[0096] The system as described above was used to form coppernanoparticles utilizing copper wire. In this example, copper was used,but other metals have also been used, with merely adjustments in theevaporation temperature and the selection of the dispersant medium.Purasolv® BL was used as the collection media at an injection rate of0.25-0.5 l/min into the vacuum pump. Argon gas flow was maintained at alevel of 10 Torr within the evaporation chamber. The nanoparticlesformed were collected at >95% efficiency in the dry screw pump at aprimary particle size of 0.01 microns.

EXAMPLE 3 Collection of Aluminum Nanoparticles in HTPB/Heptane Solution

[0097] The system as described in FIG. 2 was used to evaporate aluminumwire, form aluminum nanoparticles and collect it into an HydroxyTerminated Polybutadiene (HTPB)/heptane mixture. In this example,aluminum was used, but other metals have also been used, with merelyadjustments in the evaporation temperature and the selection of thedispersant medium. The aluminum nanoparticles formed were conveyedthrough the dry screw pump and collected in a venturi scrubber operatingbehind the pump. The nanoparticles were collected into the resin/solventslurry at an efficiency >90%. The primary particle size of the aluminumnanoparticles formed was 0.03 micron. The % of the HTPB in the heptaneslurry was 10 wt %.

[0098] With a ratio of 3 to 1 weight resin to aluminum nanoparticles,the mean agglomerate size in the resin was ˜0.25 microns. The heptanecould then be evaporated off of the resin to yield a useablenanoparticle-loaded HTPB slurry.

[0099] As shown in FIG. 1, apparatus 10, respectively, for providing thepresent invention dispersions comprise furnace 12 having thereincrucible 14 supported by electrodes 15 connected to an external powersupply (not shown) and containing vaporizable metal 16. Gas inlet tube18 allows non-reactive gas 19 to be introduced into furnace 12 toenvelop and assist in formation of fine particles 20 and facilitatetheir transportation through transfer tube 22, drawn by mechanicalvacuum pump 23 to collection vessel 24. Collection vessel 24 containsliquid dispersing medium 26 into which transfer tube 22 having tube end21 allows transported metal particles 20 and non-reactive gas 19 tobubble into medium 26 or it allows transported metal particles 20 andnon-reactive gas 19 being transported through transfer tube 22 havingtube end 21 to impinge upon medium 26 (FIG. 1). Condensor 32 is providedto return any evaporated liquid from liquid medium 26 back to collectionvessel 24. Condensor 32 is connected to trap 38 and optionalsupplemental pump 40. Supplemental pump 40 is used to evacuate entireapparatus 10 prior to and during use. Bypass valve 34 and bypass tube 36allow for facile evacuation of furnace 12 prior to onset of metal 16evaporation. Valves 42 and 44 allow isolation of apparatus 10 fromsupplemental pump 40.

[0100] Alternatively, the mechanical pump 23 and the collection vesselmay be provided in a single unit wherein the medium 26 is introducedinto the pump so that the small metal particles and gas are combinedwith the medium 26 within the pump.

[0101] Other reactor designs to provide dispersions of the invention canbe envisioned, including a rotary metal atom reactor such as describedin Metal Vapour Synthesis in Organometallic Chemistry, J. R. Blackborowand D. Young, Springer-Verlag (New York), 1979 and a spinning diskassembly such as described in Jpn. J. Appl. Phys., 13, 749 (1974), aslong as the location of the mechanical pump is after the system forgenerating the gas phase of metal particles carried in the gas phase andbetween or coincident with the condensation/collection zone. Both typesof reactors could be used to generate dispersions of metal particles.Additionally, metal that can be evaporated directly to generate discretemetal molecules may be used in these reactors to prepare dispersions ofthe present invention.

[0102] In addition to resistive heating, other means of applying heat tothe metal may be envisioned. These include laser heating, inductiveheating, plasma jet, plasma arc discharge, laser flashing, sputtering,and others known to those skilled in the art.

[0103] In a preferred embodiment the present invention provides metalnanoparticles, dispersions of metal nanoparticles, which metalnanoparticles are solid (i.e., solidified) dispersions of metalparticles in a polymer, the particles having a mean average particlesize of less than 0.1 micrometer (100 nanometers). In addition, theparticles have a narrow size distribution and the dispersions aretransparent, and are resistant to flocculation. For medical devices orlayers that provide critical physical or chemical properties, it isprudent to minimize the amount of metal particle required to achieve aspecified level of a particular secondary property to preserve thoseproperties; hence, a well dispersed nanoparticle of metal is desirable.Dispersions of ultrafine metal particles are more stable than theirlarger sized counterparts. This resistance to agglomeration prior tosolidification of the liquid polymer makes the manufacturing of aarticles less sensitive to uncontrollable environmental factors. Smallerparticles form more stable dispersions/suspensions than do largerparticles.

[0104] The dispersions of the present invention can be used to preparearticles by means of any type of article forming, such as casting,coating, toning, printing, molding, including injection molding andextrusion processes, casting, including spin casting, etc. Such articlesinclude fibers and molded articles including conductive layers,structural elements, explosive materials, composites, electromagneticresponsive or resistive materials and the like. Coated dispersions ofthe present invention can be used to prepare high quality metallicgraphic arts constructions such as sublimation type thermal transferrecording media, and any other applications where dispersed material isuseful. The coating can be accomplished by any means known in the artincluding bar coating, knife coating, thermal mass transfer, curtaincoating, meniscus coating, slot coating, etc.

[0105] A wide variety of particle coating processes are known in theart. For larger size particles, e.g., for 1 mm or greater, the simplest,most cost effective process is direct immersion of particles in acoating composition (e.g., liquid, gel, powder, etc.) and removing thecoated particles from the coating environment (with drying or agitation,as needed to fix the coating and separate the particles). Particles maybe carried on a conveyor belt and sprayed or otherwise coated withcoating compositions. Particles may be projected of dumped into adeposition coating environment (e.g., spray chamber, vacuum depositionchamber, electrostatic chamber, etc.) where the coating is applied.Mixtures of particles and coating compositions may be mixed, thensprayed to fix of dry the coating on the surface of particles.

[0106]FIGS. 2A and 2B show crucible constructions particularly useful inthe prqctice of the present invention. FIG. 2A shows a crucible 200having a main frame 202 preferably made of graphite or ceramic materialand lips 204 that extend over the support surface 206 for the metal (notshown) in the crucible 200.

[0107]FIG. 2B shows a crucible system 220 comprising and exterior sheath222 having an insert 224 within the sheath 222. The insert 224 also hasthe lips 226 extending over the support surface 228 on the insert 224.

[0108]FIG. 3 is a flow diagram depicting a generic aspect of the presentinvention.

[0109] In this application, the metallic dispersions can be placed intoa high pressure reactor and charged with the fluoromonomers at theappropriate pressure and temperature to start the polymerizationreaction which coats the metallic nanoparticles with an appropriatefluoropolymer

What is claimed:
 1. A process of collecting metal nanoparticlescomprising forming a vapor of a metal that is solid at room temperature,the vapor of the metal being provided into an inert gaseous carryingmedium, solidifying at least some of the metal within the gaseous streamto form metal nanoparticles, moving the metal nanoparticles in a gaseouscarrying environment through a dry mechanical pumping system, and whilethe metal nanoparticles are within the dry mechanical pumping system orafter the nanoparticles have moved through the dry pumping system,contacting the metal nanoparticles with an inert liquid collectingmedium.
 2. The process of claim 1 wherein the metal nanoparticlescomprise a metal having a vaporization temperature between 100° C. and3000° C.
 3. The process of claim 1 wherein the metallic nanoparticlescomprise at least one metal selected from the group consisting of Ti,Ag, Au, Pb, Sn, Zr, and Ni.
 4. The process of claim 2 wherein the inertliquid collecting comprises an organic liquid.
 5. The process of claim 3wherein the inert liquid collecting comprises an organic liquid.
 6. Theprocess of claim 2 wherein metal particles within the dry mechanicalpumping system are contacted with an inert liquid collecting medium. 7.The process of claim 3 wherein metal particles within the dry mechanicalpumping system are contacted with an inert liquid collecting medium. 8.The process of claim 2 wherein metal particles are contacted with aninert liquid collecting medium after leaving the dry mechanical pumpingsystem.
 9. The process of claim 3 wherein metal particles are contactedwith an inert liquid collecting medium after leaving the dry mechanicalpumping system.
 10. The process of claim 10 wherein the nanoparticlesare also collected by physical filtration.
 11. The process of claim 11wherein a vacuum system is installed to provide additional driving forceto collect nanoparticles by physical filtration.
 12. The process ofclaim 10 wherein the nanoparticles are collected in slurry andsubsequently coated using high pressure reactor coating.
 13. Anapparatus for providing dispersions of ultrafine metal particles havingan average size of between 0.5 and 100 nanometers comprising: a) asource of vaporized metal connected to a mechanical pump and acollection vessel, the source of vaporized metal providing an stream ofnon-reactive gas flow away from the source and towards a mechanicalpump; b) a source of metal into the source of vaporized metal; c) asource of non-reactive gas to carry metal material towards themechanical pump; d) a fluid source for a fluid to collect metalparticles and/or condense metal vapor into nanoparticles; e) amechanical pump for moving the non-reactive gas with metal material andthe fluid to collect metal particles and/or condense metal vapor intoparticles; and f) a contact zone for the i) non-reactive gas and metalmaterial and ii) the fluid to collect metal particles and/or condensemetal vapor into particles; wherein the contact zone is within themechanical pump or after the mechanical pump.
 14. The apparatus of claim13 having a source of metal and a source for a reactive gas stream toeffect reaction of metal particles.
 15. The apparatus of claim 14wherein the fluid is introduced into the mechanical pump to firstcontact the non-reactive gas with metal.
 16. The apparatus of claim 15wherein the non-reactive gas is removed from the mechanical pump aftermetal material concentration in the gas has been reduced by contact withthe fluid.
 17. The apparatus of claim 16 wherein a liquid recyclingsystem to return liquid into the mechanical pump is provided so thatrecycled liquid with particulate metal content comprises the recycledliquid.
 18. The apparatus of claim 16 wherein the source of liquidprovides an organic liquid to the mechanical pump.